linux-kernel - Re: [PATCH v2 4/5] blk-iocost: fix sleeping in atomic context warnning

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Date:   Tue, 15 Nov 2022 09:16:52 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Tejun Heo <tj@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@....de, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2 4/5] blk-iocost: fix sleeping in atomic context
 warnning

Hi,

在 2022/11/15 6:07, Tejun Heo 写道:
> On Fri, Nov 04, 2022 at 10:39:37AM +0800, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> match_u64() is called inside ioc->lock, which causes smatch static
>> checker warnings:
>>
>> block/blk-iocost.c:3211 ioc_qos_write() warn: sleeping in atomic context
>> block/blk-iocost.c:3240 ioc_qos_write() warn: sleeping in atomic context
>> block/blk-iocost.c:3407 ioc_cost_model_write() warn: sleeping in atomic
>> context
>>
>> Fix the problem by introducing a mutex and using it while prasing input
>> params.
> 
> It bothers me that parsing an u64 string requires a GFP_KERNEL memory
> allocation.
> 
>> @@ -2801,9 +2806,11 @@ static void ioc_rqos_queue_depth_changed(struct rq_qos *rqos)
>>   {
>>   	struct ioc *ioc = rqos_to_ioc(rqos);
>>   
>> +	mutex_lock(&ioc->params_mutex);
>>   	spin_lock_irq(&ioc->lock);
>>   	ioc_refresh_params(ioc, false);
>>   	spin_unlock_irq(&ioc->lock);
>> +	mutex_unlock(&ioc->params_mutex);
>>   }
> 
> Aren't the params still protected by ioc->lock? Why do we need to grab both?

Yes, the params is updated inside ioc->lock, but they can be read
without the lock before updating them, which is protected by mutex
instead.

> 
> Any chance I can persuade you into updating match_NUMBER() helpers to not
> use match_strdup()? They can easily disable irq/preemption and use percpu
> buffers and we won't need most of this patchset.

Do you mean preallocated percpu buffer? Is there any example I can
learn? Anyway, replace match_strdup() to avoid memory allocation sounds
good.

Thanks,
Kuai
> 
> Thanks.
>